Diatoms from the Calvert Formation

Today Anna Trochim and I published a Jeffersoniana on diatoms from Carmel Church and other Calvert Formation localities in Virginia (available online). This was the complete, peer-reviewed version of a poster we presented at SE-GSA last March, and it’s also based on Anna’s undergraduate thesis at Emory & Henry College.

We pulled sediment samples off of fossil whales from three different sites: a baleen whale from Westmoreland State Park, the Rappahannock River sperm whale, and “Picasso” from Carmel Church. After Anna developed a procedure for cleaning the diatoms, we examined slides from all three sites to see what was there.

The primary goal of the work was to determine the diatom biozone for each site, so that we could try to narrow down the age of the deposits. This was particularly important for Carmel Church. Carbonate fossils are not preserved at Carmel Church, so the mollusks and foraminifera that are often used for determining ages aren’t an option there. Our previous age estimates at Carmel Church were based almost entirely on land mammal biostratigraphy, combined with even less certain negative evidence (absence of Carcharocles chubutensis, absence of Squalodon) which only narrowed the date down to 14-11 million years (upper Calvert or lower Choptank Formations). Lithology suggested that we were in the Calvert rather than the Choptank, so closer to 14 than to 11.

As expected, all three of our sites had large numbers of diatoms. Moreover, our hypothesis that all three sites were approximately the same age was correct, or at least consistent with our data. Here’s Figure 2 from our paper, showing our range chart for each site:

You can see from the chart that, based on these diatoms, Carmel Church correlates to the upper part of Bed 15 of the Calvert Formation, the very top of the unit. In my earlier papers on Carmel Church, I had generally called Carmel Church Bed 14. This is because in other areas Bed 14 is more coarse than Bed 15, and of course the Calvert is quite coarse at Carmel Church. But I had always privately wondered, on very shaky evidence, if Carmel Church was a little younger. This was based on the relatively advanced shape of the fresh Carcharocles teeth, as well as details in the premolar of a peccary specimen, but I was never sure (or even confident enough to speculate in print). It was nice to see this confirmed. The coarse sediment at Carmel Church relative to other localities is probably due to its nearer-shore location.

We also got some reasonable paleoenvironmental information from the diatoms. An abundance of benthic diatoms, particularly at Carmel Church, suggests that the water was shallow, likely less than 20 m deep (diatoms are photosynthetic and require sunlight). Most of the common diatoms, and all of the most abundant taxa at Carmel Church, either prefer or are tolerant of brackish water conditions (although there was a less common taxon at Carmel Church that prefers hypersaline conditions). There was also a mix of cold water and warm water taxa at each site. That might mean we’re picking up a seasonal signal, but another possibility is that the Salisbury Embayment was at a latitude where cooler water from the north was meeting warmer water from the south.

As seems to often be the case, some of our results were so unexpected that we hadn’t even thought to look for them in advance. We suspected that all three sites were the same age, and given that they were all located in the same embayment within forty miles of each other, we expected the diatom floras to be pretty similar between all three sites. We missed the boat on this one, because the three sites are wildly different from each other!

In our study we identified a total of 39 diatom taxa, but only six of these were found at all three sites. Twenty taxa were only found at one site (13 at Carmel Church, 5 at Westmoreland, and 2 at the Richmond County). Carmel Church was easily the most diverse site, with 28 species in 19 genera. Westmoreland had 19 species in 12 genera, while Richmond County had 15 species in 12 genera. But Westmoreland had by far the greatest number of individual specimens; the slides were so rich it was sometimes difficult to pick out individual specimens.

The diversity differences were present even among very common species. Two taxa, Paralia sulcata and Melosira westii, were dominant at every site. But Coscinodiscus radiatus, present in vast numbers at Westmoreland, was absent at Carmel Church and Richmond County. Likewise, Grammatophora marina was one of the dominant species at Carmel Church but was not found at the other two sites. Below are Paralia sulcata (left) and Melosira westii (right), the only two of the 39 taxa that were dominant at every site:

These wild differences in the diatoms between these sites can as a surprise to us, and we still don’t have a good explanation. But it does give us a direction for future studies.

Reference:

Trochim, A. R. and Dooley, A. C. Jr., 2010. Diatom biostratigraphy and paleoecology of vertebrate-bearing Miocene localities in Virginia. Jeffersoniana 23:1-18.
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